Greenpeace has put up banners saying "change the politics, save the climate". But it is the scandal around expenses that will be uppermost in MPs minds on their first day back, not the climate. However, the fallout from the row could yet produce a future intake of politicians who put the needs of the planet above loyalty to party.

If the scores of candidates who are likely to replace departing MPs are selected not by their constituency party or party lists but by open primaries, then it could be a real opportunity for those who care about the climate to put up candidates.

There are already encouraging signs that this may happen. In August the Tories announced the winner of the first ever open postal vote of an entire constituency in Totnes. The result was not a career politician or one of the usual suspects but a doctor, Sarah Wollaston.

Many big hitters in the Labour party have recently shown their support for open primaries. They include Ken Livingstone, who has backed them to elect the next mayor of London and the Tottenham MP David Lammy, who has called for them in every London borough. The foreign secretary, David Miliband, has also backed the cause, arguing the case in cabinet as part of the answer to the cynicism surrounding politics and falling party membership.

For open primaries to really engage with the electorate, political parties must not just use them as a way of deflecting public anger but instead ensure they are rooted in the community and open to anyone and everyone. This comes at a cost. It is estimated that the open primary in Totnes cost the Tories about £40,000 to organise. But in the greater scheme of things this seems a small price to pay to regain the public's trust and participation in politics. Building the cost of open primaries into future discussions about the state funding of political parties and election campaigns could be an option.

If open primaries were used to replace departing MPs then there is no reason why environmental activists like those on the roof of the House of Commons wouldn't be elected to parliament, as well as people from many different walks of life. In return, Greenpeace and other environmental organisations should encourage their members to participate in the democratic process rather than shout from the rooftops.

As a former campaigner with Greenpeace I've participated in many actions and understand only too well its antipathy towards politics and politicians. But with more than five million members, green groups could be one of the major beneficiaries of such electoral reform. While many of them are constrained by rules governing charities, their members vote and many may be prepared to back candidates who put the climate at the centre of their campaign.

For example the Royal Society for the Protection of Birds has more than a million members and the National Trust has more than three and a half million members. In the case of the trust this is the equivalent to the population of Birmingham, Leeds, Glasgow and Sheffield combined. And it has already declared that it wants to become "the largest green movement in the world" and put its members in the front line against climate change. Applying this electoral muscle in open primaries could well help to swing the vote, particularly in constituencies where climate change is a big issue, such as those with nuclear or coal-fired power stations.

The Greenpeace action is timely, coming just two months before Copenhagen and as the climate talks in Bangkok having broken down in acrimony. The Committee on Climate Change has also published a report saying a "step change" in emissions cuts is needed.

Sadly, I suspect the only questions that will be asked in the House about the protest will be regarding Westminster security. Yet if parliament advocated genuinely open primaries and they were embraced by the environmental movement, the banners in future could read "join in politics, save the climate".